Process for purifying exhaust gases, especially from vacuum pyrolysis installations

ABSTRACT

A process for purifying exhaust gases, especially from vacuum pyrolysis installations by means of the extraction of the exhaust gases and their combustion with a supply of air. Immediately after their formation, the exhaust gases are subjected to complete combustion with a controlled air supply and an under pressure of 0.5 to 0.95 bar (500 to 50 mbar absolute) generated by a vacuum pump, whereupon the gaseous reaction products are cooled to 10° to 25° C. to prevent damage to the vacuum pump and then the cooled gaseous reaction products are extracted by the vacuum pump.

The invention relates to a process for purifying exhaust gases,especially from vacuum pyrolysis installations, by means of theextraction of the exhaust gases and their combustion with a supply ofair.

To date such exhaust gases have been extracted and subsequentlysubjected to their own combustion, whereupon the gaseous combustionproducts were released into the atmosphere. Depending on the exhauster,there exists the possibility on a larger or smaller scale that theexhaust gases with their possibly harmful components will have anegative impact on the exhauster, especially an operating medium used inthe exhauster, e.g. water or oil. The negative effect on the operatingmedium goes so far that the operating medium has to be disposed asspecial waste, and thus legal provisions for disposal of special waste,which have to be initiated, as a rule, with an approval process, areapplied. In any case this represents a significant complication of theprior art process.

The invention is based on the problem of designing the purification ofthe exhaust gases in such a manner that neither the exhauster nor anyoperating medium in the exhauster can be negatively effected. Thisproblem is solved by application's invention.

Preferably the exhaust gases are exhaust gases from vacuum pyrolysisinstallations, preferably toluene and/or benzene, and exhaust gases fromextrusion processes, preferably monomers, such as caprolactam, and/oroligomers, preferably the short-chained, more complex compounds ofmonomers are from polymer melts.

By means of the combustion of the exhaust gases safe combustionproducts, preferably CO₂ and/or H₂ O are produced both for the exhausterand an operating medium used therein. So that at this stage the hightemperatures of the combustion products resulting from the combustion ofthe exhaust gases cannot damage the exhauster, which is designed here asa vacuum pump, immediately following the formation of the combustionproducts (gaseous reaction products) there is a cooling step thatprovides that the gaseous reaction products are cooled so far, viz. to10° to 25° C., that they can no longer damage a vacuum pump thatfollows. The vacuum pump is preferably a liquid ring vacuum pump. In apreferred embodiment a slide vane rotary pump is used. For purposes of aslide vane rotary pump the range of cooling is expanded to 10° to 200°C., preferably 100° to 200° C. Following the combustion of the exhaustgases to innocuous, gaseous reaction products, whereby the combustiontakes places at an underpressure of 0.5 to 0.95 bar (500 to 50 mbarabsolute) that is generated by the vacuum pump, and the subsequentcooling of the gaseous reaction products, then the gaseous reactionproducts can no longer cause any damage to the vacuum pump, so that thecooled gaseous reaction products and the cooling water from the spraycondenser can be extracted with the vacuum pump and can be released intothe open air. For purposes of a slide vane rotary pump, the range ofunderpressure, which is generated by the vacuum pump, expands to 0.5 to0.99 bar (500 to 10 mbar absolute).

The aforementioned liquid ring vacuum pumps belong in general to thegroup of displacement pumps. The gas to be extracted is conveyed inthese pumps with the aid of circulating liquid, the liquid ring. Thistype of vacuum pump is suitable for conveying gases and steams ofvirtually any kind, provided a suitable liquid (i.e. operatingmedium)--water in the normal case--is chosen to form the liquid ring. Inthe present invention water is preferred.

An impeller is arranged eccentrically in a cylindrical housing filled inpart with liquid. Owing to the rotation of the impeller, the liquidforms a ring rotating concentrically to the axis of the housing. It isachieved with this arrangement that the operating liquid leavespiston-like from the wheel cells and re-enters. As a consequence of therotating impeller, these gases and steams are conveyed in the directionof rotation, whereby the cells decrease again. The gases and steams thatare pumped off are compressed and ejected from the inside of the pumpwith a portion of the operating mediums.

The heat produced with the operation of the pump is emitted by way ofthe operating medium, for which reason new operating medium has to bedelivered continuously for cooling. The mixture of operating medium andgas is separated again in a subsequent liquid separator (i.e.separator), during which process a portion of the operating medium canbe conveyed again to the vacuum pump.

In a less preferred embodiment of the invention, in which a liquid ringvacuum pump with, e.g. water or oil as operating medium is used, thegaseous reaction products are cooled after their formation to atemperature, which is higher than 25° C., e.g. 30° to 40° C. Therefore,this embodiment is less preferred, since cavitation in the pump canoccur, thus decreasing the vacuum in the pump and damaging the pump inthe long run.

The aforementioned slide vane rotary pump functions, as the name alreadysuggests, according to the slide vane rotary principle. An eccentricallymounted rotor rotates in a cylinder. By means of the centrifugal forceof the rotary motion, the slide vanes, which slide in slots in therotor, are pressed against the cylinder wall, whereby the slide vanesdivide the crescent-shaped space between cylinder and rotor intochambers. When the chambers are connected to the suction channel, thegas is sucked in, with more rotation compressed, and finally ejectedwith the operating medium into an operating medium separator andseparated there again from said separator. The operating medium,preferably oil, collects at the bottom in the separator and is injectedagain into the compression space (circulation system lubrication). Theoutgoing air which is without any operating medium is then released tothe atmosphere.

Therefore, the process according to the invention can be usedcontinuously for cleaning exhaust gases without any danger to the organscontained in it. In a less preferred embodiment the process according tothe invention is used discontinuously.

Preferably the combustion of the exhaust gases with controlled supply ofair is conducted in the case of a heavy metal and chlorine-free exhaustgas in the presence of a catalyst at temperatures ranging from 350° to500° C.; for exhaust gases with catalytic poisons such as heavy metaland chlorine compounds, thermally at temperatures ranging from 850° to1200° C. Suitable catalyst is primarily a metal substrate coated withplatinum, preferably similar to an autocatalyst, which is suitablepreferably for oxidation of aliphatic and aromatic hydrocarbons with upto 7 carbon atoms.

After cooling, the gaseous reaction products can be passed in anadvantageous manner through a separator, which separates from the gasstream specific water-soluble gases together with the operating medium,discharged from the vacuum pump, and the extracted cooling water fromthe spray condenser.

For cooling, a spray condenser or a heat exchanger can be used in anadvantageous manner, where the latter renders the heat emitted in ituseable again as energy.

The device to implement the process is designed expediently in such amanner that its individual organs are arranged in the sequencecombustion--system, cooling device and vacuum pump, where the vacuumpump receives the gaseous reaction products as already cooled and can nolonger be damaged by said reaction products.

Expediently a high temperature furnace can be used as the combustionsystem. If a catalyst is used, said catalyst can be designed expedientlyin such a manner that an air heating apparatus with heating wires isarranged in front of the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are shown in the Figures.

FIG. 1 is a device for catalytic purification of exhaust gases;

FIG. 2 is a device for purifying exhaust gases with thermal combustion;

FIG. 3 is a device for catalytic purification of exhaust gases, where aheat exchanger is provided for cooling.

The device depicted in FIG. 1 and designed for purifying exhaust gasescontains the vacuum-sealed pipe 2, through which the exhaust gases to bepurified are conveyed (see drawn arrow). Air is conveyed to the pipe 2by way of a control valve 1, in order to generate in the pipe 2 a gasmixture, which is necessary for combustion and contains an adequateamount of oxygen. The pipe 2 has the electric air heating apparatus 3,which is designed as an electric heating coil and which heats the gasmixture which is passed through up to a temperature ranging from 350° to500° C. which is necessary for the catalytic combustion. As analternative only the air conveyed by way of the control valve 1 can alsobe heated with an air heating apparatus and can be mixed with theexhaust gases to be purified (not illustrated). To avoid energy losses,the pipe 2 is provided with thermal insulation. In the direction of flowbehind the air heating apparatus 3 there is installed the temperatureprobe 9, which indicates the temperature of the gases and subject to theeffect of the control valve 1 controls and monitors the air heatingapparatus 3 in order to avoid overheating and damage to the air heatingapparatus 3 and the following catalyst 4. Thus the following catalyst 4always has optimal air temperatures in order to reach optimal combustionvalues.

The gases heated thus flow into the region of the catalyst 4, in whichthe catalytic combustion of the supplied gases takes place, thus formingchemically innocuous reaction products. Since the gas temperature risesdue to combustion, another temperature probe 10 is provided whose signalis used in a manner analogous to the signal of the temperature probe 9so that damage to the catalyst 4 and inadequate combustion of the gasesis avoided. Then the reaction products flow into the spray condenser 5,which is filled partially with fillers 11, e.g. Rasching rings, withwhich the surface acting on the supplied gases is correspondinglyraised. Water, which is atomized inside the spray condenser 5, isconveyed through a feed pipe 12 to the spray condenser 5, thus resultingin the desired cooling effect for the supplied gases. The vacuum pump 6,with which an underpressure ranging from 0.5 to 0.95 bar (500 to 50 mbarabsolute) is generated in the region of the combustion and the spraycondenser 5, is connected downstream of the spray condenser 5. Forpurposes of a slide vane rotary pump this range is expanded to 0.5 to0.99 bar (500 to 10 mbar absolute). The separator 7, which can separatefrom the gas stream the liquid, which comprises specific water-solublegases and the operating medium, discharged from the vacuum pump, andextracted cooling water from the spray condenser, is attached to theoutput of the vacuum pump 6. The mixture of gas and liquid coming intothe separator 7 is separated in such a manner that the liquid flows offwith the gases dissolved therein by way of a connecting piece 13. Gasesthat are not dissolved in the liquid are discharged by way of theconnecting piece 14. For safety reasons the temperature is measured bymeans of the measuring instrument 8, which measures the concentration ofthe unburned gases, and thus monitors and controls by way of feedbackwith the control valve 1 the portion of explosive, combustible orconvertable portions of the combustion residues.

To automate the operational sequence in the device, the measurementresults of the temperature probes 9 and 10 and the measuring instrument8 can be combined by way of a computer, which can derive from thelinkage of the measurement results a signal, which controls the controlvalve 1 in such a manner that the amount of air required for thecombustion is supplied at any time.

The device depicted in FIG. 2 involves the use of thermal combustion.Apart from that, the device according to FIG. 2 has the same componentsas the device in FIG. 1, for which reason components used in the samemanner are provided with the same reference numerals in both Figures.

The exhaust gases delivered by way of pipe 2 are led here through thehigh temperature furnace 15, which is heated by means of the electricheater 16 to a temperature ranging from 850° to 1200° C. The thermalcombustion of the exhaust gases takes place in the region of the hightemperature furnace 15; then said exhaust gases are subjected to coolingand separation, as in the case of the device according to FIG. 1. Evenwith a device according to FIG. 2 it is possible to combine themeasurement results of the measuring instrument 8 and the temperatureprobe 10, in order to derive a control signal for the control valve 1.

The device depicted in FIG. 3 is a device for catalytic purification ofexhaust gases according to FIG. 1, wherein, however, instead of thespray condenser 5, there is a heat exchanger 17 to cool the gaseousreaction products. Apart from that, the components are identical to theones shown in FIG. 1, for which reason components used in the samemanner are provided with the same reference numerals in both Figures.

In general a spray condenser or heat exchanger is suitable for coolingthe gaseous reaction products, using a liquid ring vacuum pump, whereaswith the use of a slide vane rotary pump a heat exchanger is preferred.

In the following the cooling of the gaseous reaction products and thesubsequent steps are described in detail. In a preferred embodiment, inwhich a liquid ring vacuum pump, preferably a water ring vacuum pump, isused, the reaction products, cooled in the spray condenser by means ofwater, and the water are delivered into the vacuum pump. In this type ofa vacuum pump water, preferably water that comes from the condenser,serves as the operating medium. The water is thrown by means ofcentrifugal force against the inside wall of the pump and forms on thiswall a sealing layer. In this embodiment the operating medium of thepump is mixed, therefore, without fail with the condensate products; forwhich reason in the prior art it also results in a deleterious manner inthe operating medium being polluted.

The operating medium and the reaction products are conveyed through thepump. Finally this operating medium is separated from the gaseousreaction products in the separator.

As aforementioned, the invention can also be implemented using a slidevane rotary pump. Compared to the liquid ring vacuum pumps, such pumpsoperate with a different operating medium (for sealing), preferably oil.Since such a pump exhibits a higher operating temperature, preferablyover 100°, a heat exchanger is provided preferably for cooling thegaseous reaction products. Owing to the higher operating temperature ofthe slide vane rotary pump, the gaseous reaction products existexclusively as gases, including water as a water vapor, which can bereleased to the atmosphere. The oil conveyed together with the cooledreaction products through the pump is now recovered in the separator andfed back to the pump. By recycling the oil, up to 99.9% of the oil canbe recovered.

In principle the present invention is suitable for purifying exhaustgases originating from an installation that requires a relatively highunderpressure.

It must also be pointed out that the entire purification process takesplace in a closed system, so that the purification of the exhaust gasesis conducted in a manner that is optimal for the environment. Therefore,the significance of this invention must also be judged in connectionwith the increasing efforts in the domain of environmental protection.Vacuum pyrolysis installations have required a permit pursuant to thelegislation (TA-Air, 5 Bm SchG [Technical specifications for maintainingclean air--First general administrative regulations concerning Federallaw on the protection against harmful effects on the environment]). Todate proposals for after-treatment of the substances (especiallyresidual gases) conveyed by the pump have been tried, e.g. by means ofactivated charcoal filters; however, these proposals usually fail onaccount of the required expense in proportion to the obtainable results.Thus, to date none of the past proposals promise a solution both to theexhaust gas problem and also the Water pollution.

I claim:
 1. A process for purifying and extracting exhaust gases atunderpressure conditions generated by a vacuum pump comprising:(a)mixing the exhaust gases with a controlled air supply; (b) subjectingthe mixture of exhaust gases and air of step (a) to combustion at anunderpressure ranging from about 0.5 to about 0.99 bar generated by avacuum pump to obtain gaseous reaction products; (c) cooling the gaseousreaction products before said gaseous reaction products are conveyed inthe vacuum pump to a temperature sufficient to essentially precludedamage to the vacuum pump; and (d) extracting the cooled gaseousreaction products.
 2. The process of claim 1, wherein the gaseousreaction products are cooled to a temperature between about 10° andabout 200° C.
 3. The process of claim 2 wherein the combustion takesplace at an underpressure ranging from about 0.5 to about 0.95 bar andthe gaseous reaction products are cooled to a temperature between about10° and about 25° C.
 4. The process of claims 2 or 3, wherein thecombustion is conducted at temperatures ranging from about 850° to about1200° C.
 5. The process of claim 4 wherein the cooled gaseous reactionproducts are extracted in the form of a mixture of a gas stream and aliquid, and further the process comprises:(e) separating the gas streamfrom the liquid and any soluble gases dissolved in the liquid.
 6. Theprocess of claims 2 or 3, wherein the combustion is conducted in thepresence of a catalyst at temperatures ranging from about 350° to about500° C.
 7. The process of claim 6, wherein the catalyst comprises ametal substrate coated with platinum.
 8. The process of claim 6 whereinthe cooled gaseous reaction products are extracted in the form of amixture of a gas stream and a liquid, and the process furthercomprises:(e) separating the gas stream from the liquid and any solublegases dissolved in the liquid.
 9. A system for purifying and extractingexhaust gases at underpressure conditions comprising:(a) a chamber formixing the exhaust gases and a controlled supply of air; (b) means forcombusting the exhaust gases and air mixed in the chamber to producegaseous reaction products; (c) a vacuum pump means in communication withthe mixing chamber for generating an underpressure within the systemranging from at least about 0.5 to at least about 0.99 bar; (d) means incommunication with the mixing chamber for cooling the gaseous reactionproducts to a temperature at which said products will not cause damageto the vacuum pump means; and (e) means for extracting the cooledgaseous reaction products from the cooling means.
 10. The system ofclaim 9 wherein the means for combusting the exhaust gases and air is ahigh temperature furnace.
 11. The system of claim 9 wherein the meansfor combusting the exhaust gases comprises a heating apparatus and ameans for catalytic combustion.
 12. The system of claim 11 wherein themeans for catalytic combustion includes a catalyst comprising a metalsubstrate coated with platinum.
 13. The system of claims 9, 10, 11 or 12wherein the cooling means is constructed and arranged to cool thegaseous reaction products to a temperature between about 10° and about200° C.
 14. The system of claim 13 wherein the vacuum pump means isconstructed and arranged to generate the underpressure in the range fromat least about 0.5 to about 0.95 bar and the cooling means isconstructed and arranged to cool the gaseous reaction products to atemperature between about 10° and 25° C.